During the last decades there has been a substantial global increase in the amount of carbon dioxide (CO2) emission to atmosphere. According to the Kyoto agreement and based on the precautionary principle it is important to reduce the emission of climate gases such as CO2 in order to counteract changes in climate. One way is to capture CO2 when converting energy from fossil fuel in a gas power plant and/or thermal power plant. The different elements in the CO2 value chain include technology for CO2 capture, transportation of and finally final storage or exploitation of CO2, for example for increased oil recovery from the oil reservoir (IOR).
The technology development within the field of CO2 capture from combustion processes may be divided into three main categories, wherein the following feasible and useable technology is:                Removal of CO2 prior to combustion (hydrogen power plant—Pre Combustion Type);        Removal of CO2 subsequent to combustion (exhaust gas cleaning—Post Combustion Type), and        Stoichiometric combustion of natural gas and oxygen (oxy-fuel type).        
Of the three main categories, the exhaust gas cleaning technology is the most developed technology, and there exists plants running on an intermediate scale. Exhaust gas cleaning has still a cost reduction potential and may most quickly be tested in a pilot plant.
The present technology for exhaust gas cleaning is based on absorption of carbon subsequent to combustion. A gas power plant of this type is disclosed in the prior art literature, textbooks and publications.
Emission of CO2 may usually be reduced by 85-90% compared to a plant without any exhaust gas cleaning system. The exhaust gases from a standard combined cycle gas turbine plant contain ca. 3.5 volume % of CO2 and the exhaust gas must be cooled down to normal operation temperature for amine washing, such temperature being approximately 40-44° C. In an atmospheric adsorption tower the CO2 in the gas is transferred to the liquid phase by chemical absorption in the amine liquid. It is imperative to have a large area of contact between the gas and the liquid. Consequently, the tower may have to be as high as 30 metres or more. For a gas power plant of 400 MW the volume of exhaust gas to be cleaned is in the order of 2.500.000 Nm3/hr, and the required cross sectional area of the absorption column will be 260-320 m2.
In the regenerating plant the CO2 is removed from the amine liquid by heating the mixture up to 120-125° C. Steam from the gas power plant is used both for heating, diluting and transportation of CO2 out of the plant. Subsequent to cooling and condensation, CO2 and water is separated, using a desorption column in order to obtain mass transfer from liquid to steam. The desorption column may have a height of ca. 20 metres and having a cross sectional area of 60-150 m2.
The amine liquid solution may then be re-used for absorption subsequent to retrieval of heat from the liquid solution and reduction of the temperature. The desorption process produces a waste which has to be handled separately. For a 400 MW gas power plant said waste represents approximately 90-1500 tonnes/year, out of which 30-500 tonnes/year are amines, salts and organic carbon. The regeneration process requires substantial amount of energy, resulting in a reduction in efficiency of approximately 20% for a standard gas power plant producing electricity. A standard gas power plant having this type of exhaust gas cleaning means has thus the disadvantage that both investment costs and running costs are significantly high. In addition, such plants require large areas.
More cost effective CO2 capture plants are known for other type of utilization, such as for example for CO2 capture from a well stream from a natural gas field, e.g. the Sleipner field in the North Sea or from synthesis gas production. The operation of such plants are, however, subjected to completely different conditions of operation, i.e. higher pressure and/or higher CO2 content, compared to exhaust gas cleaning from plants commonly operating at atmospheric pressure and with low CO2 content of approximately 3.5 volume %.
In the North Sea there exists production platforms using an amine absorption plant and a desorption plant, but such CO2 capturing plants are of a substantially smaller size, less costly and requiring less energy to operate.
Further it is known to use CO2 membranes, for example of the polymer type, for pressurized well streams and process gases in plants, especially in USA. Said plants are even more energy- and cost effective. Contrary to the amine plants the CO2 membrane plants require no liquid parts and consequently no regeneration of liquid, thereby avoiding the requirement for additional energy.
Another advantage is the feasibility of operating at a higher temperature, typical 40-100° C. in the CO2 membrane plant. A certain leakage flow of N2 may escape through the CO2 membrane, and N2 may be removed subsequent to being compressed to a pressure where CO2 is transformed to liquid, allowing N2 to be separated. In case of injection into an oil reservoir for increasing the oil recovery it may not necessary to separate N2 and CO2. In certain cases it may be advantageous to maintain a mixture of N2 and CO2 in order to increase oil recovery.
Capturing plants of the amine type for exhaust gas having CO2 enriched content and/or at a higher pressure are disclosed in several documents, such as for example WO 95/21683 or WO 00/57900.
In order to obtain lower NOx emission from incinerators, it is proposed to recycle cooled flue gas by means of a booster fan back to the combustor.